The invention relates to a computed tomography method wherein a radiation source which emits a conical radiation beam performs a relative motion in the form of a helix relative to an examination zone. The invention also relates to a computed tomography apparatus and to a computer program for carrying out such a method.
A computed tomography method of this kind is known from PCT/IB 99/00027 (PHQ 98-020). Therein, a CT image is reconstructed from the acquired measured values by applying the filtered backprojection method wherein the measured values are subjected to one-dimensional ramp-like filtering after a so-called parallel rebinning operation. The filter data associated with rays having passed through a given point in the examination zone is subsequently summed; this yields the attenuation coefficient for the rays of the radiation beam in the relevant point.
Despite the attractive quality of the CT (CT=Computed Tomography) image thus formed, it may still contain image artefacts, notably when the conical radiation beam has a large angle of aperture in the direction perpendicular to the axis of rotation and parallel thereto.
It is an object of the present invention to realize a further enhancement of the image quality offered by a method of the kind set forth. This object is achieved by means of a computed tomography method which includes the steps of:
(a) generating a conical radiation beam which contains a plurality of rays that emanate from a radiation source and traverse an examination zone or an object present therein,
(b) generating a relative motion between the radiation source on the one side and the examination zone or the object on the other side, which relative motion includes a rotation about an axis of rotation and a displacement relative to the axis of rotation and is shaped as a helix,
(c) acquiring measured values, using a detector unit, which are dependent on the attenuation of the rays in the examination zone during the relative motion,
(d) calculating link values by summing the measured values along links of a network in a three-dimensional parameter space describing the position and orientation of the rays,
(e) filtering the link values in order to produce filter data for links that are associated with rays that pass through a given surface of the examination zone,
(f) calculating the attenuation of the radiation in pixels on the surface by summing the filter data of links which approximate the trajectory that is defined in the parameter space by the rays that pass through the relevant pixel,
(g) repeating at least the steps e) and f) for other surfaces that are mutually offset in the direction of the axis of rotation.
The invention is based on the recognition of the fact that the artefacts occurring in the known method are due to the fact that the assembly of measured values subjected to a common (one-dimensional and ramp-like) filter operation changes from one filter operation to another. In accordance with the invention, however, the various filter operations are performed only on measured values that are associated with rays that pass at least approximately through one and the same surface in the examination zone.
Limiting the filter operations to these measured values is possible because a two-stage summing operation is performed in conformity with the characteristics d) and f) and because the filter operation in conformity with the characteristic e) is inserted between these two steps.
Claim 2 discloses a preferred version of the invention. All rays passing through the surface defined in claim 2 are situated in planes which extend parallel to the axis of rotation and together define an angular range of exactly 180xc2x0. Therefore, the attenuation in this surface can be reconstructed without utilizing redundant measured data. For all other surfaces it would be necessary to take into account rays from a larger angular range.
It is to be noted that a publication in the name of the inventor in the magazine xe2x80x9cInternational Journal of Imaging Systems and Technologyxe2x80x9d, Vol. 11, 2000 (pp. 91-100) discloses a method for filtered backprojection in which the one-dimensional filtering (preceding the backprojection) involves the measured values of rays that pass through a plane that contains the axis of rotation along inclined lines. These lines approximate the projection of a surface which is referred to as Pi-surface (being identical to the surface defined in claim 2) onto the plane. However, the projection is line-shaped for a few projection directions only. Therefore, optimum image quality is not achieved when the attenuation is reconstructed each time two-dimensionally for a series of such surfaces. Moreover, the filtering and the backprojection therein take place in the customary order, while in the present invention these two steps are interleaved comprising the above three steps d), e), and f).
The further version disclosed in claim 3 is advantageous notably when the detector unit has comparatively large dimensions in the direction of the axis of rotation. It can be dispensed with in the case of smaller dimensions, because the cosine function then suitably approximates the value 1 for all rays.
When the attenuation values are acquired at the grid points of a regular Cartesian grid in conformity with claim 4, CT images of arbitrary surfaces in the examination zone can be simply formed at a later stage.
Seemingly, the larger the angular range wherefrom measured values are acquired for the calculation of link values before the filtering, the more specifically are these data related to one Pi-surface only. On the other hand, a requirement for not filtering original projection data in the customary way is that the links are straight line segments. The larger the angular range, the less accurate is the piece-wise linear approximation of pixel-trajectories in measurement space. A suitable compromise between these contradictory requirements is offered by the version disclosed in claim 5.
The rays are advantageously subjected to so-called parallel rebinning in conformity with claim 6.
Claim 7 describes a computed tomography apparatus which is suitable for carrying out the method in accordance with the invention and claim 8 discloses a computer program for executing the reconstruction method in accordance with the invention.